前程百利小编为考生带来关于火星上是否存在生命的托福阅读材料。人类一直在进行火星探索，对于火星上的大气、水分等都有一些研究结果。作为天文学热门话题，火星在大家的备考中怎么能轻易放过?所以希望大家通过这份材料，了解更多关于火星的背景知识。
The current understanding of planetary habitability—the ability of a world to develop and sustain life—favors planets that have liquid water on their surface. This most often requires that the orbit of a planet lie within the habitable zone which for the Sun currently extends from just beyond Venus to about the semi-major axis of Mars.During perihelion Mars dips inside this region but the planet’s thin (low-pressure) atmosphere prevents liquid water from existing over large regions for extended periods. The past flow of liquid water however demonstrates the planet’s potential for habitability. Recent evidence has suggested that any water on the Martian surface would have been too salty and acidic to support terrestrial life.
The lack of a magnetosphere and extremely thin atmosphere of Mars are a greater challenge: the planet has little heat transfer across its surface poor insulation against bombardment and the solar wind and insufficient atmospheric pressure to retain water in a liquid form (water instead sublimates to a gaseous state). Mars is also nearly or perhaps totally geologically dead; the end of volcanic activity has stopped the recycling of chemicals and minerals between the surface and interior of the planet.
Evidence suggests that the planet was once significantly more habitable than it is today but whether living organisms ever existed there is still unclear. The Viking probes of the mid-1970s carried experiments designed to detect microorganisms in Martian soil at their respective landing sites and had some apparently positive results including a temporary increase of CO2 production on exposure to water and nutrients. However this sign of life was later disputed by many scientists resulting in a continuing debate with NASA scientist Gilbert Levin asserting that Viking may have found life. A re-analysis of the now 30-year-old Viking data in light of modern knowledge of extremophile forms of life has suggested that the Viking tests were also not sophisticated enough to detect these forms of life. The tests may even have killed a (hypothetical) life form.Tests conducted by the Phoenix Mars Lander have shown that the soil has a very alkaline pH and it contains magnesium sodium potassium and chloride.The soil nutrients may be able to support life but life would still have to be shielded from the intense ultraviolet light.
At the Johnson space center lab some curious shapes have been found in the Martian meteorite ALH84001. Some scientists propose that these geometric shapes could be fossilized microbes extant on Mars before the meteorite was blasted into space by a meteor strike and sent on a 15 million-year voyage to Earth. However an exclusively inorganic origin for the shapes has also been proposed.
Small quantities of methane and formaldehyde recently detected by Mars orbiters are both claimed to be hints for life as these chemical compounds would quickly break down in the Martian atmosphere.It is possible that these compounds may instead be replenished by volcanic or geological means such as serpentinization.
以上就是前程百利小编为大家带来的托福阅读材料，平时的阅读练习是托福考试阅读备考的重要一环。希望大家利用好每份材料，尽可能多地从中获取知识。预祝大家托福考试试取得好成绩。
